Growth Hormones :
The term 'hormone' was coined first by Starling (1906) in animal physiology. The internal factors that influence growth are called growth hormones or growth regulators as they inhibit, promote or modify the growth. Growth promoters are auxins, gibberellins (GA) and cytokinins (CK). Growth inhibitors in plants are ethylene and abscissic acid (ABA). All phytohormones are growth regulators.
According to Thimann and Pincus (1948) "Plant hormones are organic substances produced naturally in higher plants affecting growth or other physiological functions at a site remote from its place of production and active in very minute (optimum) amount". Hormones are transported through phloem parenchyma (Phillips 1971).
a. Auxins (Auxien = to grow):
F. W. Went in 1931, used this term first. Auxin was isolated from urine of a person suffering from Pellagra (Kogl and H. Smit 1931). In plants, it is synthesized in growing tips or meristematic regions of plants from where it is transported to other plant parts. The most common and important natural auxin is Indole-3-acetic acid (IAA). Tryptophan is the primary precursor of IAA in plants. It is the first hormone to be discovered in plants and is primarily responsible for cell elongation. It shows polar transport - Basipetal transport in stem. Now synthetic auxins like IBA (Indole butric acid), NAA (Naphthalene acetic acid), 2, 4-D pichloro (Phenoxy acetic acid), etc. are used.
The auxin is the first hormone to be discovered in plants. Discovery of auxins dates back to 19th century when Charles Darwin (1886) was studying tropism in plants. He exposed canary grass coleoptile to unilateral light. He concluded that a growth stimulus is developed in the coleoptile tip and transmitted downwards to the growth zone. This has caused bending of the tip towards light.
The Danish plant physiologist Boysen - Jensen (1910) cut off the colepotile and inserted thin plate of gelatin beween the tip and the cut stump. He observed that coleoptiles tip still bends towards unilateral light.
Paal (1919) cut off the tip of colepotile and replaced it asymmetrically on the cut coleoptile stump. He observed that the colepotile tip bent away from the side bearing tip even in dark. F.W. Went (1928) successfully isolated natural auxin from Avena coleoptile tips. He cut off the tip and placed them on small agar blocks. Then after certain period of time placed the agar blocks asymmetrically on cut coleoptile stump that caused bending. He demonstrated the presence of substance which could diffuse into agar blocks. Went named this substance as auxin.
Physiological effects and applications of auxin:
The primary effect is cell enlargment. In most of the higher plants, growing apical bud inhibits the growth of lateral buds. This is called as apical dominance. Auxin stimulates growth of stem and root. Auxin induces multiplication of cells, hence used in tissue culture experiments to produce callus. It stimulates formation of lateral and adventitious roots. These are marketed as synthyetic herbicides. e.g. 2, 4-D (2,4 dichlorophenoxy acetic acid). It kills dicot weeds without affecting monocot crop plants.
The seedless fruits like orange, lemon, grapes, banana etc. are produced by application of auxin (i.e. induced parthenocarpy). Auxins promote cell division in cambium and also cause early differentiation of xylem and phloem. It promotes root elongation in low concentration and shooting at higher concentration. It also hastens early rooting in propagation by 'cutting'. Foliar spray of NAA and 2,4-D induces flowering in litchi and pineapple. Likewise, it prevents premature fruit drop in apples, pear and oranges, and also prevents formation of abscission layer. Auxins play a role in elongation of cell. It is known to increase rate of respiration. Auxins break dormancy in seed and promote quick germination.
